A. Technical Field
The present invention relates to phase, frequency-locked and similar loops. Although such loops as phase and frequency-locked loops commonly find application in data communications systems, their use is not limited to communication systems and the present invention may be applied in other areas with equal advantage.
B. Problems in the Art
Phase and frequency-locked loops (or locking loops referred to in the following application are meant to be either frequency or phase locked loops unless otherwise specified) are used in many applications, including communication systems, to demodulate signals. The locking loop demodulation scheme may be employed whether the signals are digital or analog, base-band or band-pass, narrow-band or spread spectrum. Locking loops use a feedback mechanism to acquire and track transmitted signals. For demodulation, the received signal is routed to a phase or frequency detector which develops a signal related to the difference between the received signal's phase and a local, reference signal's phase. The difference, or error, signal thus developed is fed back to control the frequency and/or phase of the reference signal. This feedback mechanism permits the loop to acquire and track the transmitted signals.
Before a loop can track a signal, it must first acquire the signal. The acquisition range of a loop is, typically, considerably greater than the tracking range but, especially under conditions of low signal-to-noise ratio, the loop's acquisition time rapidly increases with initial frequency error. This increase in acquisition time represents loss of channel capacity; and any reduction in acquisition time translates directly to a corresponding increase in valuable channel capacity.
Additionally, it is desirable to extend the acquisition range of a phase or frequency locked loop since that permits accommodation of a greater variation in signal frequencies. Such variations may be due to oscillator drift and to Doppler shift in mobile systems, the combination of which can amount to a significant fraction of a system's symbol rate. That is, those frequency shifts, if not compensated for, could lead to false negative or false positive symbol recognition.
Loop parameters can be adjusted to accelerate acquisition of a signal frequency, but, unfortunately, those adjustments result in signal-to-noise degradation. If loop parameters could be adjusted for fast acquisition, then, when the loop is close to locking, readjusted to provide an optimum signal-to-noise ratio for tracking and normal operation, a great improvement over current systems would be realized.
There have also been other attempts at reducing loop acquisition time. One method for deriving control of loop parameters is based on a Phase-Locked-Loop's (PLL's) frequency error. This method, however, puts severe limits on the adjustment of the loop parameters--limits which are greatly reduced by the method of the current invention.
Other approaches to improve Locking Loop performance employ spectral analysis of the received signal to produce a coarse estimate of the offset between the received signal and the reference signal. The reference signal is then adjusted to reduce this offset. This approach is very compute-intensive though, requiring additional hardware and expense. Further, the method is limited by the computational speed of the additional hardware.
An aiding loop such as an automatic frequency control (AFC) loop can be added to a PLL to accelerate the PLL's signal acquisition. However, the added gain element of the AFC contributes additional noise to the received signal. It is important, therefore, to be able to manipulate the AFC gain in such a way as to both accelerate acquisition when the PLL is not locked and to maximize the signal-to-noise ratio when the PLL is locked. In order to achieve these goals, a good estimate of the AFC's proximity to lock is needed. Further, a method of employing this estimate to properly control the AFC loop parameters to achieve these diverse goals is also needed.
It is therefore an object of the present invention to improve the capture performance of locking loops such as phase locked loops or frequency locked loops while maintaining the signal-to-noise ratio of a signal which is introduced to the locking loop.
It is a further object of the present invention to establish these improvements in a cost effective fashion.
These and other objects, features and advantages will become more apparent with reference to the accompanying specification and claims.